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LIQUID-LIQUID EXTRACTION: POSSIBLE ALTERNATIVE TODISTILLATIONJames R. Fair a & Jimmy L. Humphrey aa The University of Texas at Austin , Austin, Texas, 78712Published online: 27 Sep 2010.
To cite this article: James R. Fair & Jimmy L. Humphrey (1984) LIQUID-LIQUID EXTRACTION: POSSIBLE ALTERNATIVE TODISTILLATION, Solvent Extraction and Ion Exchange, 2:3, 323-352
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SOLVENT EXTRACTION AND ION EXCHANGE, 2(3), 323-352 (1984)
LIQUID-LIQUID EXTRACTION: POSSIBLE ALTERNATIVE TO DISTILLATION
James R. F a i r and Jimmy L. Humphrey The U n i v e r s i t y o f Texas a t Aus t i n
Aus t in , Texas 78712
ABSTRACT
L i q u i d - l i q u i d e x t r a c t i o n can o f t e n be a v i a b l e a l t e r n a t i v e t o d i s t i l l a t i o n f o r t h e separa t ion o f l i q u i d mixtures. While i t does n o t enjoy t he support o f confirmed, r e l i a b l e methods f o r scaleup and design, as i s t h e case f o r d i s t i l l a t i o n , i t does o f f e r a p o t e n t i a l f o r energy reduc t ion and a l s o can handle temperature- l a b i l e mater ia ls . The purpose o f t h i s paper i s t o p rov ide an overv iew o f e x t r a c t i o n p r i n c i p l e s and a p p l i c a t i o n s t h a t should be u s e f u l f o r conceptual design o f new processes under development.
INTRODUCTION
The concept o f separa t ing one o r more components from a l i q u i d
m i x t u r e by means o f s e l e c t i v e so lven t e x t r a c t i o n has been known
and p r a c t i c e d f o r decades, even cen tu r ies . I n con t r as t t o
d i s t i l l a t i o n , however, t h e technology f o r ana l ys i s and design o f
e x t r a c t i o n processes has been slow t o develop. E x t r a c t i o n was not
i n c l uded w i t h t h e o r i g i n a l chemical eng ineer ing u n i t opera t ions
and d i d not f i n d a p lace i n t he f i r s t e d i t i o n o f Chemical
Engineers ' Handbook (1934). I n t he 1936 e d i t i o n o f Elements o f
Chemi c a l Engineer ing, by Badger and McCabe, know1 edge o f
e x t r a c t i o n was sumnari zed as f o l l ows :
Copyright O 1984 by Marcel Dekker, Inc.
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324 FAIR AND HUMPHREY
[Ex t r ac t i on ] i n vo l ves opera t ions t h a t a re i n wide use no t o n l y i n chemical eng ineer ing b u t a l so i n o t he r a r t s . . . . The t heo ry I s q u i t e inadequate, and few impor tan t q u a n t i t a t i v e s t ud i es have ever been made. Consequently, t he apparatus has developed a long l i n e s d i c t a t e d by convenience and experience, r a t h e r than by a t h e o r e t i c a l ana l ys i s o f t h e problem. . . . When t h e book Absorp t ion and E x t r a c t i o n was w r i t t e n by
Sherwood i n 1937, o n l y one o f e i g h t chapters d e a l t w i t h
e x t r a c t i o n . A s i m i l a r p r o p o r t i o n o f coverage was found i n t h e
second e d i t i o n o f Chemical Engineers ' Handbook (1941). It i s
c l e a r t h a t t h e more q u a n t i t a t i v e aspects o f e x t r a c t i o n technology
have been developed w i t h i n t h e l a s t f o r t y years o r so. It seems
c l e a r a l s o t h a t chemical engineers p r e f e r no t t o use e x t r a c t i o n
i ns tead o f , say, d i s t i l l a t i o n because o f t h e i r g rea te r f a m i l i a r i t y
w i t h t h e l a t t e r opera t ion and because methods f o r scaleup and
des ign o f e x t r a c t i o n processes are much l e s s r e l i a b l e than they
a re f o r o t he r separa t ion processes such as d i s t i l l a t i o n .
Desp i t e what might be termed neg lec t o f e x t r a c t i o n by those
who develop chemical eng ineer ing technology, t h e process does ho ld
promise t o lower energy requirements f o r separa t ing some l i q u i d
mixtures. One can v i s u a l i z e a so l ven t c o n t a c t i n g ope ra t i on
c a r r i e d ou t a t ambient temperature and pressure, i n which a minor
amount o f a h i g h - b o i l i n g m a t e r i a l i s . s e l e c t i v e l y removed f rom a
l a r g e q u a n t i t y o f more v o l a t i l e ma te r i a l . I n d i s t i l l a t i o n it
would be necessary t o supply cons iderab le l a t e n t heat o f
v a p o r i z a t i o n t o c a r r y ou t such a separat ion. For t h e e x t r a c t i o n
case i t would be necessary t o vapor ize o n l y t he minor amount o f
e x t r a c t e d ma te r i a l . S i t u a t i o n s o f t h i s t ype a re o f t e n encountered
i n process design. A more q u a n t i t a t i v e d i s cuss i on o f them i s
g i ven l a t e r i n t h e p resen ta t ion .
The purpose o f t h i s paper i s t o p rov ide t h e reader w i t h a
general rev iew o f t he s t a t e o f t he a r t o f t he technology f o r t he
ana l ys i s and design o f e x t r a c t i o n processes. L i m i t a t i o n s i n space
p rec lude a d e t a i l e d examinat ion o f any o f t he p a r t s o f t h a t
technology. It i s hoped t h a t t h e reader w i l l o b t a i n some
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ALTERNATIVE TO DISTILLATION
a p p r e c i a t i o n o f t he p lace o f l i q u i d - l i q u i d e x t r a c t i o n i n t h e t o t a l
scheme o f separa t ion methods use fu l f o r t he separa t ion o f l i q u i d
m i x t u res a t commercial scales o f operat ion. Al though e x t r a c t i o n
can be app l i ed t o s o l i d - l i q u i d processes, as imp l i ed by t h e t i t l e
o f t h e paper, coverage here i s l i m i t e d t o l i q u i d - l i q u i d systems.
CURRENT APPLICATIONS
One should no t conclude a t t h i s p o i n t t h a t e x t r a c t i o n has
remained as a l a b o r a t o r y c u r i o s i t y f o r l i q u i d m i x t u re separat ion.
Some very impor tan t and la rge-sca le processes use ex t r ac t i on , and
perhaps t he bes t known o f these i s t he one t h a t produces a l a r g e
p a r t o f t h e Un i ted S ta tes ' p roduc t ion o f h i g h - p u r i t y benzene. I n
t h i s process an aromat ic m i x t u re con ta i n i ng benzene, to luene, and
xy lene ( c a l l e d t h e "BTX" f r a c t i o n ) i s separated from c l ose -bo i l i n g
p a r a f f i n s and naphthenes i n t he product from re fo rming operat ions.
The r e f i n i n g o f l u b r i c a t i n g o i l s i s another tonnage e x t r a c t i o n
process. Usefu l h igher-valued f u e l s a re be ing ex t r ac ted from
heavy r es i dua l o i l s from r e f i n e r y c rack i ng operat ions. E x t r a c t i o n
i s w ide l y used i n t h e food and pharmaceutical i n d u s t r i e s ; a modern
example o f such use i s t he removal o f c a f f e i n e from c o f f e e beans
by t h e use o f s u p e r c r i t i c a l carbon d i o x i d e solvent . Many more
examples o f e x t r a c t i o n a p p l i c a t i o n s cou ld be mentioned, f o r
example i n t h e nuc lear i ndus t r y , bu t t he above should p rov i de some
pe rspec t i ve f o r t he reader.
DEFINITIONS
The s imp les t e x t r a c t i o n system comprises t h ree components:
t h e so l u te , o r t h e ma te r i a l t o be ex t rac ted ; t he so lven t , which
must not be complete ly m i s c i b l e w i t h t he o the r l i q u i d s ; and t h e
" c a r r i e r , " o r nonsolute p o r t i o n o f t he feed m i x tu re t o be
separated. For t h e case o f coun te rcur ren t ex t r ac t i on , t h e f l ows
o f these m a t e r i a l s are shown i n Fig. 1. It should be noted t h a t
d i s t i n c t i o n must be made between t h e l i g h t phase and t h e heavy
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EXTRACT FEE0 LIGHT SOLVENT B+C (+A1 A.C
A "CARRIER" B = SOLVENT C = SOLUTE
( Distributed Component I
B A (+B+C) 'SOLVENT RAFFINATE
SOLVENT RAFFINATE HEAVY SOLVENT
B A (+C+B) A = "CARRIER" B = SOLVENT C = SOLUTE
(D is t r i bu ted Component I
B+C (+A1 A+C EXTRACT FEED
FIGURE 1. E x t r a c t i o n no ta t ion .
phase, between t h e dispersed and t h e cont inuous phase, and
between t h e r a f f i n a t e phase and t h e e x t r a c t phase. The t e rm ina l
streams from an e x t r a c t o r a re t he e x t r a c t and t h e r a f f i n a t e . Note
a l s o t h a t t h e l o c a t i o n o f t he p r i n c i p a l i n t e r f a c e depends upon
which phase i s dispersed.
A t y p i c a l e x t r a c t i o n system i s shown i n Fig. 2. As con t ras ted
w i t h t he s imple systems o f Fig. 1, t he feed stream i s shown
e n t e r i n g t h e e x t r a c t i o n column toward t he center , and t he column
i s p rov ided w i t h r e f l u x i n t h e form o f e x t r a c t product t h a t has
been separated from the solvent . While e x t r a c t i o n arrangements
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RAFFINATE C
f
- FEED
+ MAKEUP SOLVENT -
E X T R A C T REFLUX
FIGURE 2. E x t r a c t i o n system, w i t h e x t r a c t r e f lux .
can d i f f e r , depending upon t h e system t o be separated as w e l l as
t h e type o f equipment t o be used, t h e arrangement i n Fig. 2 i s
presented t o enable t h e d i r e c t comparison w i t h , say, a r e b o i l e d
abso rp t i on system. There i s indeed a d i r e c t analogy between
e x t r a c t i o n and absorpt ion. The s t r i p p e r , which i s a d i s t i l l a t i o n
column, i s an impor tan t p a r t o f t h e e x t r a c t i o n system, and i t can
consume l a r g e amounts o f energy i f t h e p r o p o r t i o n o f s o l u t e i n t h e
feed i s h igh and i f a s i g n i f i c a n t amount o f e x t r a c t r e f l u x i s
needed.
PHASE EQUILIBRIUM
The s imp les t t e rna r y system i s shown i n Fig. 3; t h i s i s c a l l e d
Type I, f o r one immisc ib le pa i r . For such a system t h e c a r r i e r
and t h e so lven t a re e s s e n t i a l l y immisc ib le , wh i l e t h e c a r r i e r -
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(Solute)
A Type I System
A f B immiscib le
''A Richm "B R i c h "
R a f t inate E a t roct Phose ( E Phose)
B ( C a r r i e r 1 / ( S o l v e n t
Tie L ine
FIGURE 3. Phase diagram, Type I system.
s o l u t e and so l ven t - so l u te p a i r s are misc ib le . The diagram shows a
s ingle-phase reg i on and a two-phase region; f o r e x t r a c t i o n t o be
f eas i b l e , composi t ions must be such as t o f a l l w i t h i n t h e two-
phase envelope.
Phase e q u i l i b r i u m r e l a t i o n s h i p s a re i n d i c a t e d i n Fig. 3; t he
t i e l i n e s connect e q u i l i b r i u m phase composi t ions and thus p rov i de
a bas is f o r s e l e c t i v i t y :
('C P A ) e x t r a c t phase , = ( ' c / ' A ) r a f f i n a t e phase
which w i l l be recognized as t h e separa t ion f a c t o r equ i va l en t t o
r e l a t i v e v o l a t i l i t y i n d i s t i l l a t i o n . Thus, BCA = K C / K A . where
t h e e q u i l i b r i u m r a t i o K i s
A f i n a l p o i n t regard ing t he Type I system: t h e p l a i t p o i n t shown
i n Fig. 3 i s t he i n t e r s e c t i o n o f t h e r a f f i n a t e phase and e x t r a c t
phase boundary curves, and no separa t ion can be made a t t h a t
po i n t . The analogy i s w i t h t he azeotrope composi t ion i n
d i s t i l l a t i o n .
F ig. 4 shows another t ype o f t e r n a r y l i q u i d - l i q u i d system, one
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A 6 C miscible
A
( C a r r i e r I ( S o l v e n t I
FIGURE 4. Phase diagram, Type I 1 system.
i n which t h e r e a re i m m i s c i b i l i t i e s between so l ven t and so lu te , and
between so l ven t and c a r r i e r ( thus, Type 11). The t i e l i n e s are
i nd i ca ted , and t h e r e i s no p l a i t po in t . With t h i s t ype o f system
i t i s poss i b l e t o o b t a i n an e x t r a c t t h a t i s e s s e n t i a l l y f r e e o f
c a r r i e r , a s i t u a t i o n t h a t i s no t poss i b l e w i t h t h e Type I system
shown i n Fig. 3. There a re a l so a few Type 111 systems, i n which
i m m i s c i b i l i t i e s e x i s t among a l l t h r e e pa i r s , b u t such systems are
r e l a t i v e l y r a r e i n e x t r a c t i o n system design. For a l l systems,
temperature i n f l uences t h e l o c a t i o n s o f t h e phase envelopes, and a
norma l l y immisc ib le system can become complete ly m i s c i b l e i f t h e
temperature i s r a i sed s u f f i c i e n t l y .
Re1 i a b l e 1 i qu i d -1 i q u i d e q u i l i b r i u m da ta a re c r u c i a l t o t h e
r a t i o n a l and economic design o f e x t r a c t i o n processes. Such da ta
can be measured w i t h l e s s d i f f i c u l t y than can v a p o r - l i q u i d
e q u i l i b r i a ; t h e phases a re brought t o e q u i l i b r i u m i n a s u i t a b l e
con ta i ne r and then al lowed t o separate complete ly be fo re they are
sampled f o r analys is .
Many e q u i l i b r i a have been pub l i shed and should be consu l ted
n o t on ly f o r poss i b l e use i n scale-up bu t a l so f o r c a l i b r a t i n g
equipment f o r l abo ra to r y measurements. References 1 t o 8 e i t h e r
p rov i de d i r e c t data o r f u r n i s h guidance t o t h e l i t e r a t u r e where
t h e da ta can be found. Treybal [9] has l i s t e d r ep resen ta t i ve
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FAIR AND HUMPHREY
C o m p o s i t e e x t r a c t
F Feed r o f f i n o t e
Solvent F x t r o r l - . . . . - - , Net f l ow to le f t . R-S SF-E = A
FIGURE 5. Crosscur ren t and c o u n t e r c u r r e n t e x t r a c t i o n .
va lues o f t h e e q u i l i b r i u m r a t i o [Eq. ( 2 ) ] f o r 278 systems. I n
genera l , t h e da ta r e p o r t e d have no t been sub jec ted t o a
thermodynamic cons is tency a n a l y s i s , and a l lowance f o r e r r o r s
shou ld be made.
STAGE CALCULATIONS
It i s convenient t o model e x t r a c t i o n processes on an
e q u i l i b r i u m stage bas is , even i f t h e equipment operates i n a
c o u n t e r c u r r e n t mode ( a s i n packed columns). For s i n g l e - s t a g e
e x t r a c t i o n s , a m i x e r - s e t t l e r arrangement i s used, w i t h t h e s t i r r e d
vessel des igned t o p r o v i d e a c l o s e approach t o e q u i l i b r i u m . For
m u l t i p l e - s t a g e e x t r a c t i o n s , bo th c r o s s c u r r e n t and c o u n t e r c u r r e n t
arrangements may be used, as i n d i c a t e d i n Fig. 5. The
c o u n t e r c u r r e n t system i s more usual and i s more e f f i c i e n t i n i t s
use o f so lvent . The stages a re arranged as i n d i s t i l l a t i o n , and
i t i s o f t e n convenient t o use d i s t i l l a t i o n - t y p e equipment such as
t r a y columns and packed towers.
There i s a minimum s o l v e n t r a t e t h a t corresponds t o t h e
minimum r e f l u x r a t i o i n d i s t i l l a t i o n . A t t h i s r a t e , an i n f i n i t e
number o f stages would be r e q u i r e d t o make a g iven separat ion.
F ig . 6 i l l u s t r a t e s t h e minimum r a t e f o r a s imp le Type I system,
and a l s o i l l u s t r a t e s t h e g r a p h i c a l techn ique f o r s t e p p i n g o f f
e q u i l i b r i u m stages. For a g iven feed F and s o l v e n t R, va lues o f
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ALTERNATIVE TO DISTILLATION 331
FIGURE 6. Minimum so l ven t r a t e , i n f i n i t e stages.
FIGURE 7. Higher-than-minimum so l ven t r a t e , f o u r stages.
t h e e x t r a c t E and r a f f i n a t e R can be ob ta ined g r a p h i c a l l y , as
shown. When e x t r a c t and feed a re i n e q u i l i b r i u m ( t h e l i n e
connect ing them co inc ides w i t h a t i e l i n e ) , the r e s u l t i n g so lven t
r a t e i s t h e minimum ra te . I f t h e so lven t - to - feed r a t i o i s then
inc reased (segment increases i n p ropo r t i on t o segment W), stages can be stepped o f f , us i ng t h e d i f f e r e n c e p o i n t as t h e
p i v o t , as shown i n Fig. 7.
Since feed and e x t r a c t pass each o ther on t h e bottom stage,
e q u i l i b r i u m may l i m i t the p u r i t y o f t h e e x t r a c t (on a so l ven t - f r ee
bas is ) . To inc rease t h i s p u r i t y , e x t r a c t r e f l u x may be used, as
shown i n Fig. 8. The so lven t s t r i p p e r must be used i n any case,
i n o rder t h a t t h e s o l u t e may go on t o i t s in tended use and t h e
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FAIR AND HUMPHREY
FIGURE 8. Use o f e x t r a c t r e f l u x .
so l ven t r e t u r n t o t he ex t r ac to r . Re tu rn ing a p o r t i o n o f the
e x t r a c t i s equ i va l en t t o r e f l u x i n g i n d i s t i l l a t i o n , ' and i t permi ts
a much pu re r s o l u t e t o be ex t rac ted . F ig. 8 shows a l so how t he
analogy t o heat i n b o i l u p i s prov ided by t he so l ven t i n
e x t r a c t i o n .
Another g raph ica l approach t o stage de te rmina t ion embodies
p l o t t i n g t h e c a r r i e r - s o l u t e on a so l ven t - f r ee basis . Fig. 9 shows
t h e e q u i l i b r i u m r e l a t i o n s h i p s f o r t y p i c a l Type I and Type 11
systems. Note t he analogy o f t he e x t r a c t and r a f f i n a t e phases t o
t h e vapor and l i q u i d phases i n d i s t i l l a t i o n . A t y p i c a l s tage
count f o r t he Type I system i s a l s o shown i n t he f i gu re . The
so l ven t - f r ee p l o t i s e s p e c i a l l y use fu l f o r Type I 1 systems and has
a complete analogy t o t h e y-x McCabe-Thiele diagram f o r
d i s t i l l a t i o n . The e f f e c t o f r a f f i n a t e and e x t r a c t r e f l u x i s seen
r e a d i l y , and t he cen te r area feed l o c a t i o n i s e a s i l y handled.
Fo r systems o f more than t h r e e components, a n a l y t i c a l
approaches are requi red. It i s poss ib le , bu t not p r a c t i c a l , t o
handle a four-component system on a three-d imensional p l o t .
Rigorous- type models have been developed f o r hand l ing
mult icomponent systems and r e q u i r e computers f o r t h e i r so lu t ion .
I n general, these models a re p rop r i e t a r y , bu t a d i s cuss i on o f
t h e i r approach has been pub l i shed by Scheibel [lo]. It appears
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LTERNATIVE TO DISTILLATION
C = solute A = carrier
FIGURE 9. So lven t - f ree e q u i l i b r i u m diagram, t y p i c a l stages.
t h a t mult icompenent models a re no t used very o f ten , p a r t l y because
o f t he l ack o f r e l i a b l e multicomponent phase e q u i l i b r i a and p a r t l y
because t h e systems i n d i c a t i n g t h e i r use are p o o r l y def ined. It
i s o f t e n s a t i s f a c t o r y t o use a pseudoternary system, w i t h pseudo-
components represen t ing t he p r o p e r t i e s o f t he design e x t r a c t and
r a f f i n a t e streams.
SOLVENT SELECTION
The optimum so lven t f o r a g iven separa t ion i s determined from
a cons ide ra t i on o f severa l c r i t e r i a . It i s impor tan t t o note t h a t
t h e "bes t " so l ven t f o r t he l abo ra to r y o r p i l o t p l a n t development
may not be f e a s i b l e f o r t h e commercial p lan t . Some general
c r i t e r i a f o r so l ven t se l ec t i on , as o u t l i n e d by Treybal [ll , 123,
a re these:
1. S e l e c t i v i t y . A h igh value o f t he separa t ion f a c t o r enables
fewer stages t o be used.
2. E q u i l i b r i u m r a t i o . A h igh value of BCA permi ts lower
s o l v e n t l f e e d r a t i os .
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FAIR AND HUMPHREY
RAFFINATE
Q- <
FINAL EXTRACT SOLVENT
EXTRACT - HEAVY PHASE
RAFFINATE . LIGHT PHASE
C . COALESCER
S SETTLER (decanter
FIGURE 10. Two-stage m i x e r - s e t t l e r system.
Densi ty . A h igh dens i t y d i f f e r e n c e between e x t r a c t and
r a f f i n a t e phases permi ts h i ghe r c a p a c i t i e s i n equipment.
I n s o l u b i l i t y o f so lvent . I f t h e so l ven t i s t oo so l ub l e i n t h e
r a f f i n a t e , s i g n i f i c a n t so lven t losses can occur.
Recoverabi l i t y . It i s des i r ab l e t o make a c lean separa t ion o f
e x t r a c t a n t and so lven t i n the s t r i p p e r , w i t hou t excessive
energy requirements.
I n t e r f a c i a l tens ion. Low i n t e r f a c i a l t ens i on a i ds d i spe rs i on
b u t h i nde rs s e t t l i n g and phase separat ion.
T o x i c i t y , f l ammabi l i t y . These are important occupat ional
h e a l t h and s a f e t y cons idera t ions .
Cost. An e x c e l l e n t so lven t , based on l abo ra to r y t es t s , may - n o t be commercia l ly a v a i l a b l e o r may represent a very l a r g e
i n i t i a l cos t f o r charg ing t h e system. I n a d d i t i o n , losses
occur i n opera t ing systems and must be replaced.
There a re approaches t o so lven t se l ec t i on , a t l e a s t f o r
p r e l i m i n a r y screening, t h a t are based on chemical s t r u c t u r e and
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ALTERNATIVE TO DISTILLATION
i n t e r a c t i o n s between t he chemical species involved. Such
approaches have been discussed r e c e n t l y by Robbins [13].
EXTRACTION DEVICES
A g rea t many d i f f e r e n t devices a re used commercial ly; many o f
them are p r o p r i e t a r y and r e q u i r e t h e involvement o f t he p r o p r i e t o r
i n t h e scale-up procedure. A bas ic t ype o f device i s the
m i x e r - s e t t l e r system, use fu l i f on l y a few t h e o r e t i c a l stages are
r equ i r ed bu t tend ing t o be expensive i f throughputs a re high. A
diagram o f a m i x e r - s e t t l e r system i s shown i n Fig. 10. As shown.
a cons iderab le amount o f p i p i n g and pumping i s r equ i r ed f o r t he
stages, bu t by s u i t a b l e mixer design 100% stage e f f i c i e n c y can be
approached. Compact m i x e r - s e t t l e r systems, w i t h s imple
over f low-under f low arrangements, can be designed t o min imize space
and piping/pumping requirements. The m i x e r - s e t t l e r concept
suggests a general cau t i on i n e x t r a c t o r design: i n t ense a g i t a t i o n
t o p rov ide h i g h r a tes o f mass t r a n s f e r and c l ose approaches t o
100% stage e f f i c i e n c y can lead t o l i q u i d - l i q u i d d i spe rs i ons t h a t
a re d i f f i c u l t t o s e t t l e i n t o t he d i s t i n c t phases. Thus, some
balance between i n t e n s i t y o f d i spe rs i on and t ime o f s e t t l i n g must
be reached.
Many e x t r a c t o r s are o f t h e tower type, and examples o f t h i s
t y p e a re shown i n Fig. 11. The s imp les t o f these, and t he l e a s t
e f f i c i e n t , i s the spray ex t r ac to r .
The spray e x t r a c t o r comprises a v e r t i c a l vessel w i t h t he on l y
i n t e r n a l dev ice be ing a d i s t r i b u t o r f o r t he phase t o be dispersed.
As shown, t h e so lven t i s t he heavy phase and i s being d ispersed
through a pe r f o ra ted p i pe d i s t r i b u t o r . The e x t r a c t phase drops
f a l l through t h e continuous phase ( r a f f i n a t e ) , and t h e s o l u t e
d i f f u s e s f rom the cont inuous phase t o t h e d ispersed phase. The
spray e x t r a c t o r i s inexpensive, bu t i t s u f f e r s a low e f f i c i e n c y
f o r two reasons: t he re i s cons iderab le back-mixing i n t h e
cont inuous phase, thus lower ing t h e a v a i l a b l e concent ra t ion
d r i v i n g f o r c e f o r d i f f u s i o n , and t h e l a c k o f re - fo rmat ion o f drops
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FAIR AND HUMPHREY
Spray Perforated Plate
Liaht Phase Liaht Phase Heavy Phase - Inlerface
dispersed
UNAGITATED EXTRACTOR COLUMNS
Rotat ing Scheibel Reciprocotinq
Contactor Plate
MECHANICALLY AGITATED EXTRACTOR COLUMNS
FIGURE 11. R e p r e s e n t a t i v e c o l u m n - t y p e e x t r a c t o r s .
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ALTERNATIVE TO DISTILLATION 337
pena l i zes t he o v e r a l l r a t e o f mass t rans fe r . (Experiments show
t h a t a m a j o r i t y o f the t o t a l mass t r a n s f e r occurs du r i ng drop
fo rmat ion ; thus a device t h a t causes coalescence/ format ion severa l
t imes has a mass t r a n s f e r r a t e advantage.) Measurements show t h a t
spray e x t r a c t o r s do not norma l l y produce more than two o r t h ree
t h e o r e t i c a l stages.
The pulsed e x t r a c t o r become popular i n t h e mid-1950s. l a r g e l y
through exper ience w i t h sma l l -sca le un i t s . The p u l s i n g a c t i o n i s
designed t o c rea te f requent renewals o f t he i n t e r f a c i a l sur face,
thereby enhancing mass t r a n s f e r rates. Per fo ra ted p l a t e s i n t he
column min imize depar tures from e f f e c t i v e coun te rcur ren t f l o w o f
t h e phases. When scaled up t o l a r g e s izes, t h e pu lse column was
found t o s u f f e r i n e f f i c i e n c y because o f t he d i f f i c u l t y o f
p ropagat ing t h e pulses. To c o r r e c t f o r t h i s , t h e e f f e c t o f
p u l s i n g was obta ined by moving t he p l a t e s i n a r e c i p r o c a t i n g
fashion. Thus, the pu lse column i s t y p i f i e d today by t h e Kar r
e x t r a c t o r [14], which con ta ins a s e r i e s o f pe r f o ra ted p l a t e s
(w i t hou t downcomers o r upcomers) on one o r more sha f t s , w i t h t h e
assembly be ing g iven a r e c i p r o c a t i n g movement.
Represen ta t i ve performance da ta f o r two r e c i p r o c a t i n g p l a t e
columns a re g iven i n Fig. 12, taken from t h e paper by Kar r and
Lo [15] and based on t he o -xy lene /ace t i c ac i d lwa te r system. The
impor tan t design va r i ab l es appear t o be l eng th o f s t r oke ("double
ampl i tude" ) and r e c i p r o c a t i n g speed. Height equ i va l en t t o a
t h e o r e t i c a l s tage (HETS) values i n d i c a t e s tage e f f i c i e n c i e s o f t he
o rde r o f 5 t o 10%; however, i t i s poss i b l e t o use a very low t r a y
spacing (one i nch i n t he example shown) and s t i l l ma in ta i n
r e l a t i v e l y h i gh throughputs. A poss ib l e shortcoming o f t h i s t ype
o f device, o t he r than i t s cost , i s t he f a c t t h a t t he r e c i p r o c a t i n g
mot ion o f t he d r i v e r tends t o g i ve maintenance problems.
The Scheibel column, marketed under t he name York-Scheibel
column, i s designed t o s imu la te a s e r i e s o f m i x e r - s e t t l e r
e x t r a c t i o n u n i t s , w i t h se l f - con ta i ned mesh-type coalescers a t each
c o n t a c t i n g stage. The d ispersed phase holdup and mass t r a n s f e r
e f f i c i e n c y a re c o n t r o l l e d p r i m a r i l y by t he speed o f t he ag i t a t o r s .
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FAIR AND HUMPHReY
0 1 36 WATER WATER 1 1 425 T 2 36 WATER XVLENE 1 1 442 o 3 3 WATER WATER 1 1 424 x 4 3 WATER WATER 1/2 1 424
FIGURE 12. E f f i c i e n c y of t he r e c i p r o c a t i n g p l a t e e x t r a c t o r [15]. System: o - xy l ene lace t i c ac id lwate r .
Typ i ca l da ta f o r a Scheibel column [16] are shown i n Fig. 13 f o r
t h e same system represented i n Fig. 12, t he o - xy l ene lace t i c
ac i d lwa te r system. Th is system i s considered a " d i f f i c u l t " system
f o r mass t r a n s f e r because o f i t s r e l a t i v e l y h igh i n t e r f a c i a l
tens ion ; f o r "easy" systems t h e stage e f f i c i e n c y can exceed .loo%. The reason t he apparent l i m i t a t i o n o f e q u i l i b r i u m can be exceeded
i s t h a t a Scheibel stage i s i n r e a l i t y two stages--one f o r
a g i t a t i o n and one f o r coalescence. The minimum HETS f o r t he data
o f Fig. 13 i s about 13 cm and i s lower (i.e., e f f i c i e n c y h i ghe r )
when t he a c e t i c a c i d ( s o l u t e ) i s t r a n s f e r r e d from t h e aqueous
phase t o t h e hydrocarbon phase. Data i n Fig. 12 show t h e same
e f f e c t o f t r a n s f e r d i r e c t i o n on HETS. F lood ing o f t h e column
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ALTERNATIVE TO DISTILLATION
100 7
0 5 10 15
Rota t ing S p e e d . REV/=
FIGURE 13. E f f i c i e n c y o f t he Scheibel column e x t r a c t o r [16]. System: o - xy l ene lace t i c ac id lwate r .
would be reached a t about 15 r e v l s f o r t he t o t a l l i q u i d throughput
( r a f f i n a t e phase p l u s e x t r a c t phase) shown. Al though moderately
expensive, t h e Scheibel column gives very h i gh con tac t i ng
e f f i c i e n c y .
The r o t a t i n g d i s k con tac to r (RDC) was in t roduced i n t he 1950s
by t he She l l companies [ I 7 1 and has been used ex tens i ve l y i n t h e
petro leum i n d u s t r y f o r e x t r a c t i o n s i n v o l v i n g hydrocarbon systems.
~ o t d r s on a c e n t r a l s h a f t c rea te d i spe rs i on and movement o f the
phases, w h i l e s t a t o r s p rov ide t he coun te rcur ren t s taging. L i k e
t h e Scheibel u n i t , t he ROC e f f ec t i veness can be c o n t r o l l e d t o some
e x t e n t by va r y i ng t h e speed o f r o t a t i o n o f t he d i s k d ispersers.
The s i eve t r a y e x t r a c t o r resembles a s ieve t r a y d i s t i l l a t i o n
column. Downcomers ( o r upcomers) a re p rov ided t o move t h e
cont inuous phase downward o r upward, depending on whether i t i s
t h e heavy phase o r t h e l i g h t phase. Tray p e r f o r a t i o n s p rov i de f o r
drop fo rmat ion a t each stage, thus a i d i n g t h e mass t r a n s f e r
process. The s i eve t r a y dev ice i s nonpropr ie ta ry , b u t because
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FAIR AND HUMPHREY - CONTINUOUS PHASE
C- DISPERSED PHASE /
FIGURE 14. Flows i n a s i e v e t r a y e x t r a c t i o n column.
t h e r e i s r e l a t i v e l y l i t t l e pub l i shed i n f o r m a t i o n on i t s
performance a t t h e l a r g e scale, eng ineer ing f i r m s w i t h exper ience
on such performance tend t o p l a y t h e r o l e o f p r o p r i e t o r . A
diagram o f a s i e v e t r a y column s e c t i o n i s shown i n F ig . 14.
The s i e v e t r a y e x t r a c t o r i s amenable t o mechan is t i c model ing
b y chemical engineers, and approaches t o t h e model ing have been
g iven by Ske l land and Conger [18], Treybal [12], and Schulz and
P i l h o f e r [19]. The approach t o 100% stage e f f i c i e n c y i s governed
b y t h r e e mechanisms, o c c u r r i n g i n sequence:
1. Format ion o f drops a t t h e s i e v e t r a y p e r f o r a t i o n s . As no ted
above, a s i g n i f i c a n t amount o f t h e t o t a l mass t r a n s f e r occurs
d u r i n g t h i s process. Fac to rs govern ing t r a n s f e r r a t e i n c l u d e
s u r f a c e c r e a t e d (drop s i z e ) , i n t e r f a c i a l tens ion , w e t t a b i l i t y
o f t h e t r a y m a t e r i a l by t h e d ispersed phase, r a t e o f drop
f o r m a t i o n ( f l o w r a t e through t h e p e r f o r a t i o n s ) .
2. R ise o f drops through t h e c r o s s f l o w i n g cont inuous phase. Mass
t r a n s f e r r e s i s t a n c e s i n s i d e and o u t s i d e t h e drops must be
considered, and t h e approach t o f r e e r i s e v e l o c i t y o f t h e
drops must be taken i n t o account. D is tance o f drop t r a v e l , a
f u n c t i o n o f t r a y spacing, i n f l u e n c e s t h e amount o f mass
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ALTERNATIVE TO DISTILLATION
system : toluene/ acetone/water
FIGURE h e i g h t C201.
DN = hole diameter D p = drop diameter WN = hole velocity
10 20 30 40
Height H. cm
15. Dependence o f p o i n t e f f i c i e n c y o f s i n g l e drops on t he o f r i s e f o r fo rmat ion a t h igh ho le v e l o c i t i e s , w i t h j e t t i n g
t r ans fe r red , as shown i n Fig. 15, taken from t h e paper by
P i l h o f e r [20].
3. Coalescence o f drops under t he t r a y above. The c o n t r i b u t i o n
o f t h i s mechanism t o t he t o t a l mass t r a n s f e r process i s
u s u a l l y q u i t e small .
I n t h e preceding d iscuss ion o f s i eve t r a y mechanisms, and i n
t h e diagram o f Fig. 14, t he l i g h t phase i s dispersed. I f i t i s
d e s i r a b l e t o d isperse t he heavy phase, t he t r a y arrangement can be
i nve r t ed , w i t h t he cont inuous phase f l ow ing through upcomers and
t h e d ispersed phase forming drops a t t he pe r f o ra t i ons , which then
f a l l t o t he t r a y below, o f t e n a t t a i n i n g a f r e e - f a l l v e l o c i t y .
Packed e x t r a c t o r s are designed on bases t h a t are analogous t o
those f o r g a s - l i q u i d packed columns. Packing m a t e r i a l s a re t he
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FAIR AND HUMPHREY
LIGHT PHASE OUT
interface
heavy phose distributor
pocking holddown gr id
pocking support/ phose redis l r i bulor
L l G H T
packing holddown grid
l ight phase disperse,
HEAVY PHASE O U T m- FIGURE 16. Diagram of a packed ex t r ac to r .
same (e.g., I n t a l o x saddles, P a l l r i ngs , ordered packings of the
gauze o r mesh type) , and equ iva len t devices a re used f o r phase
d i s t r i b u t i o n and c o l l e c t i o n . A diagram o f a packed e x t r a c t o r i s
shown i n F ig. 16.
The maximum ra tes o f phase f lows a re ob ta ined from f l o o d
c o r r e l a t i o n s such as t h a t o f Nemunait is e t a l . 1213. T rans fe r
u n i t requirements a re computed i n s tandard fashion, and emp i r i ca l
methods a re used t o determine t r a n s f e r u n i t he igh ts . Examples o f
expe r imen ta l l y determined he igh t s o f t r a n s f e r u n i t s are shown i n
F ig. 17, from t h e paper by Nemunaitis e t a l . The te rm HOR r e f e r s
t o t h e h e i g h t o f an o v e r a l l t r a n s f e r u n i t , based on concent ra t ions
i n t h e r a f f i n a t e phase. Such i n f o rma t i on i s used t o determine
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ALTERNATIVE TO DISTILLATION
Continuous Phase Velocity, m l h r
U0'7.5 m / s
0- 0 5 0 100 150 2 0 0 2 5 0 3 0 0
Cont inuous Phose Veloci ty , f t /h r
Continuous Phase Velocity. m/hr 15 3 0 45 6 0 75 9 0
- o 5 0 IOO 1 5 0 2 0 0 2 5 0 3 0 0
Cont inuous Phose Veloci ty . f t / h r
FIGURE 17. He igh ts o f t r a n s f e r u n i t s f o r kerosene/MEK/water system, 25-mm metal P a l l r i n g s , 0.46-m column. 1.5-m packed h e i g h t C211.
r e q u i r e d h e i g h t of packing:
where t h e number o f t r a n s f e r u n i t s NOR i s c a l c u l a t e d on a b a s i s
e q u i v a l e n t t o t h a t f o r e q u i l i b r i u m s ta tes .
Some guidance i n e x t r a c t o r s e l e c t i o n may be ob ta ined f rom
Tab le 1, taken f rom t h e paper by Todd [22]. Not a l l of t h e
c u r r e n t l y a v a i l a b l e e x t r a c t i o n dev ices a re i n c l u d e d i n t h e tab1 e.
F ig . 18, f rom t h e same source, shows approx imate areas of
a p p l i c a t i o n .
Tab le 2 shows approx imate c a p a c i t y and e f f i c i e n c y da ta f o r
severa l t ypes o f e x t r a c t o r s , drawn i n p a r t f rom t h e book by Laddha
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FAIR AND HUMPHREY
Table 1. RATINGS OF SEVERAL COMMERCIAL EXTRACTORS
Contac to r : Spray B a f f l e Packed RDC Pulsed Mixer C e n t r i - P l a t e P l a t e S e t t l e r f u g a l
C a p i t a l Cost
Opera t ing & Main- tenance c o s t
E f f i c i e n c y
T o t a l Capac i t y
F l e x i b i l i t y
Vo lumet r i c E f f i c i e n c y
V e r t i c a l Space
F l o o r Space
A b i l i t y t o Hand1 e Systems That E m u l s i f y
5 = d e s i r a b l e ; 1 = u n d e s i r a b l e
Source: Reference 22.
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ALTERNATIVE TO DISTILLATION
Tab le 2. APPROXIMATE EFFICIENCY AND CAPACITY CAPABILITIES OF EXTRACTION COLUMNS
'D + 'C HETS Stages/ E~
Con tac to r (m/hr) (m) meter ( h r - I )
Spray 15-75 3.0 -6.0 0.3-0.15 3-7
Sieve 3-60 0.3 -1.8 0.5-3.30 1-120
Packed 6-45 0.9 -3.0 0.3-1.00 1-27
K a r r 18-70 0.2 -0.6 1.6-6.00 17-200
RDC 18-40 0.15-0.6 1.6-6.60 22-180
Schei be1 15-30 0.3 -0.6 1.6-3.30 29-60
and Degaleesan [23]. The e f f i c i e n c y c r i t e r i o n i s
combined phase stages per 'L = ('D ' ' C ) ( H k ) = ( f l o w r a t e ) ( u n i t h e i g h t ) ' ( 4 )
where
Note t h a t EL i s n o t a f r a c t i o n o r a percentage.
For t h e s i e v e t r a y e x t r a c t o r , o v e r a l l e f f i c i e n c y i s p r e d i c t e d
w e l l by t h e e m p i r i c a l r e l a t i o n s h i p o f Treybal [Ill:
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FAIR AND HUMPHREY
Stages
EASY HA
Difficulty of Dispersion
low phase density -high phase density di f ference difference
low interfacial-high inter fac ia l tension tension
FIGURE 18. Areas of a p p l i c a t i o n of e x t r a c t i o n dev ices [22].
where
t h e o r e t i c a l stages Eo = o v e r a l l e f f i c i e n c y = actual trays ZT = spac ing between t r a y s , m
o = i n t e r f a c i a l tens ion , mN/m
uD, uC as d e f i n e d f o r Eq. ( 4 )
An e a r l i e r rev iew o f e x t r a c t i o n equipment has been g iven by
Mor re lo and Po f fenberger [24]. A recen t and e x c e l l e n t t rea tment
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ALTERNATIVE TO DISTILLATION
DIF in Distillation Process
TS = REQUIRED HEATING MEDIUM TEMPERATURE FOR
DISTILLATION COLUMN
TSE = REQUIRED HEATING MEDIUM TEMPERATURE FOR EXTRACTION
SOLVENT STRIPPER = 600' F (316' C) RD = REQUIRED REFLUX R A T I O FOR DISTILLATION
FIGURE 19. E x t r a c t i o n vs. d i s t i l l a t i o n se l ec t i on , n o n v o l a t i l e so l ven t [27].
o f commerc ia l ly a v a i l a b l e e x t r a c t o r s i s t h a t o f Lo [25]. For
s i e v e t r a y column design an e a r l y a r t i c l e , s t i l l u se fu l from a
p r a c t i c a l s tandpo in t , i s t h a t o f May f i e l d and Church [26].
ENERGY CONSIDERATIONS
At t h e ou t se t o f t h i s p resen ta t i on i t was noted t h a t f o r some
m ix tu re separa t ion problems e x t r a c t i o n can have energy consumption
advantages over d i s t i l l a t i o n . With e x t r a c t i o n , t h e major cos t o f
energy i s f o r t he so lven t s t r i p p e r . The energy r equ i r ed f o r
d i s p e r s i n g t he phases (mechanical o r pressure) i s low i n
comparison. Thus, as f o r absorp t ion o r s t r a i g h t d i s t i l l a t i o n , t h e
energy ana l ys i s deals p r i m a r i l y w i t h a d i s t i l l a t i o n step.
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FAIR AND HUMPHREY
D/F in Distillation Process
HEATING MEDIUM TEMPERATURES FOR DISTILLATION AND EXTRACTION SOLVENT STRIPPING ARE EQUAL;
EF UX R A T I O FOR SOLVENT-EXTRACT SEPARATION '?RE\ = REFLUX R A T I O FOR SOLVENT-RAFFINATE
FIGURE 20. E x t r a c t i o n vs . d i s t i l l a t i o n s e l e c t i o n , d i f f i c u l t s o l v e n t separa t ion [271.
Comparisons o f energy requirements f o r severa l separa t ion
processes have been p rov ided by N u l l [27]. Whi le i t i s c l e a r l y
d i f f i c u l t t o make gene ra l i za t i ons . N u l l has presented gu ide l i nes
t h a t are o f d e f i n i t e i n t e r e s t t o process engineers. H i s
comparisons between d i s t i l l a t i o n and e x t r a c t i o n a re shown i n
F igs. 19 and 20. The cond i t i ons f o r these f i g u r e s represent
extreme cond i t ions .
Fo r Fig. 19, a comple te ly n o n v o l a t i l e so lven t i s assumed.
Th i s so l ven t does no t contaminate t he r a f f i n a t e and requ i r es o n l y
s imp le f l a s h s teps f o r separa t ion from the ex t r ac t . As an example
use o f t h e f i g u r e , i f 60% o f t he feed would be taken overhead i n
d i s t i l l a t i o n (D/F = 0.6) and t h e r equ i r ed hea t i ng medium
temperature f o r d i s t i l l a t i o n i s 149' C (300' F), any i n d i c a t e d
d i s t i l l a t i o n r e f l u x r a t i o s (RD) g rea te r than 2.0 would suggest
cons i de ra t i on o f e x t r a c t i o n as a v i a b l e a l t e r n a t i v e .
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ALTERNATIVE TO DISTILLATION 349
Fig . 20 covers t he oppos i te extreme where two so l ven t
s t r i p p e r s would be needed, one f o r t he so l ven t - ex t r ac t separa t ion
and one f o r a s o l v e n t - r a f f i n a t e separat ion. I f each o f these
s t r i p p e r s r equ i r ed a r e f l u x r a t i o R E o f 2.0, and f o r t he same
D/F = 0.6 i n d i s t i l l a t i o n , e x t r a c t i o n would m e r i t cons i de ra t i on i f
t h e d i s t i l l a t i o n r e f l u x r a t i o was g rea te r than 4.0. For Fig. 20,
t h e temperature o f t h e hea t i ng medium f o r so l ven t s t r i p p i n g i s
assumed t o be t he same as t h e temperature o f t h e hea t i ng medium
f o r d i s t i l l a t i o n , an extreme s i t u a t i o n , and not a l i k e l y one.
Any comparisons o f d i s t i l l a t i o n and e x t r a c t i o n must t ake i n t o
account t h e s t a t e o f t he a r t on equipment scale-up and design.
One can be much more con f i den t i n d i s t i l l a t i o n than i n e x t r a c t i o n
because o f t h e much l a r g e r amount o f development work done i n
d i s t i l l a t i o n . There has been no e x t r a c t i o n equ i va l en t o f
F r a c t i o n a t i o n Research, Inc., i n t h e area o f l a rge - sca le equipment
performance t e s t i n g . However, f o r those cases c l e a r l y i n d i c a t i n g
a s u p e r i o r i t y o f e x t r a c t i o n as a separa t ion process, reasonable
al lowances f o r unknown scale-up f a c t o r s may overcome t h e apparent
economic penal t i e s o f proceeding w i t h d i s t i l 1 a t i o n as t he se lec ted
method.
CONCLUSIONS
The technology o f e x t r a c t i o n process design and development
has advanced m a t e r l a l l y du r i ng t h e pas t few decades. A g rea t deal
o f work has been done on l i q u i d - l i q u i d e q u i l i b r i a , b u t r e l i a b l e
and general p r e d i c t i v e methods a re s t i l l no t a v a i l ab le; d i r e c t
exper imenta t ion i s s t i l l needed f o r a l l bu t t he r e l a t i v e l y s imple
systems. Methods a re a v a i l a b l e f o r computing t h e o r e t i c a l stages
o r t r a n s f e r u n i t s , once t h e e q u i l i b r i a a re i n hand. The design o f
t h e e x t r a c t i o n devices, however, remains mos t l y i n t h e p r o p r i e t a r y
a r t , and p r a c t i c e i s o f t e n dependent on t h e p r o p r i e t o r s f o r
scale-up and design in fo rmat ion . Th is i s a s i t u a t i o n t h a t needs
co r rec t i ng .
It i s l i k e l y t h a t f o r many app l i ca t i ons , a s imp le and
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350 FAIR AND HUMPHREY
nonp rop r i e t a r y e x t r a c t i o n dev ice such as a c ross f l ow s i eve t r a y
column o r a packed column would s u f f i c e . To a i d i n t h e
development o f re1 i a b l e models f o r such devices, l a r g e r - s c a l e
performance data a re needed. Such data f o r d i s t i l l a t i o n columns
have been p rov ided by F r a c t i o n a t i o n Research, Inc.; a s i m i l a r
under tak ing f o r e x t r a c t i o n would be welcome.
F i n a l l y , t h e r e appear t o be many s i t u a t i o n s i n which
e x t r a c t i o n would be l e s s energy i n t e n s i v e than d i s t i l l a t i o n .
Improvement i n t h e s t a t e o f t h e e x t r a c t i o n technology would enable
e x p l o i t a t i o n o f such s i t u a t i o n s .
ACKNOWLEDGEMENT
Th is work was supported by t he Center f o r Energy Studies, The
U n i v e r s i t y o f Texas a t Austin.
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352 FAIR AND HUMPHREY
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Received by Editor
September 26, 1983
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